Contrasting anatomical and biochemical controls on mesophyll conductance across plant functional types

Mesophyll conductance (g (m)) limits photosynthesis by restricting CO(2) diffusion between the substomatal cavities and chloroplasts. Although it is known that g (m) is determined by both leaf anatomical and biochemical traits, their relative contribution across plant functional types (PFTs) is still unclear. We compiled a dataset of g (m) measurements and concomitant leaf traits in unstressed plants comprising 563 studies and 617 species from all major PFTs. We investigated to what extent g (m) limits photosynthesis across PFTs, how g (m) relates to structural, anatomical, biochemical, and physiological leaf properties, and whether these relationships differ among PFTs. We found that g (m) imposes a significant limitation to photosynthesis in all C(3) PFTs, ranging from 10–30% in most herbaceous annuals to 25–50% in woody evergreens. Anatomical leaf traits explained a significant proportion of the variation in g (m) (R (2) > 0.3) in all PFTs except annual herbs, in which g (m) is more strongly related to biochemical factors associated with leaf nitrogen and potassium content. Our results underline the need to elucidate mechanisms underlying the global variability of g (m). We emphasise the underestimated potential of g (m) for improving photosynthesis in crops and identify modifications in leaf biochemistry as the most promising pathway for increasing g (m) in these species.